Probe device usable in measuring stress

ABSTRACT

A probe device for use with stress-measuring apparatus and having plural probes provided with mutually and, in one form of the invention, variably spaced tips. These tips preferably comprise parallel, chisel-contact tips. Transducers such as piezoelectric crystals are respectively operatively associated with said tips, with the intercoupling areas of the crystals and coaxial cable conductors coupled thereto being constructed to provide maximum shielding of such areas against external electromagnetic-wave or other interference. The rearward face of each crystal is connected by highly flexible conductor means to the central conductors of the coaxial cables. The forward faces of the respective crystals are preferably grounded through the tips and the jackets electrically connected thereto to the coaxial shields of the cables. Central members or plugs are employed to connect the shield to its respective jacket and at the same time isolate the crystal-connector junction areas.

United States Patent [72] Inventor Ben L. Seegmiller Salt Lake City, Utah [21] Appl. No. 834,834 [22] Filed June 19, 1969 [45] Patented May 25, 197] [73] Assignee The University of Utah Salt Lake City, Utah [54] PROBE DEVICE USABLE IN MEASURING STRESS 2 Claims, 6 Drawing Figs.

[52] U.S. Cl 73/67.6, 73/71.5 [51] Int. Cl G0ln 29/00 [50] Field of Search 73/67- -67.9, 70, 71.1, 71.2, 71.4, 71.5, (Inquired); 310/82, 8.3, (lnquired) [56] References Cited UNITED STATES PATENTS 1,930,905 10/1933 Nicolson 73/88.5X 3,014,364 12/1961 Crooks 73/67.8X 3,101,608 8/1963 Benson..... 73/67.5 3,345,863 10/1967 Hen 73/67.5 3,350,582 10/1967 Attwood.......... 310/8.1 FOREIGN PATENTS 732,083 6/1955 England 73/7l.5 1,132,106 10/1968 England 73/71.5

OTHER REFERENCES Variation of Elastic Wave Velocity with Frequency in Fused Quartz and Armcolron D. S. Hughes and J. M. Kennel- Journal of Applied Physics Vol. 26, No. 11 November, 1955- pp. 1307- 1309 Zero Drive A New Zero Impedance Signal Conditioning Concept for Piezo-Electric Transducers" J. E. Judd and E. P. Moran- Instr. Soc. of Amer. Reprint Pl3-2-Phymmid- 67 rec din Office 12-3-68. released 9-67.

Primary ExaminerRichard C. Queisser Assistant Examiner-C. E. Snee, III Attorney-M. Ralph Shaffer ABSTRACT: A probe device for use with stress-measuring apparatus and having plural probes provided with mutually and, in one form of the invention, variably spaced tips. These tips preferably comprise parallel, chisel-contact tips. Transducers such as piezoelectric crystals are respectively operatively associated with said tips, with the intercoupling areas of the crystals and coaxial cable conductors coupled thereto being constructed to provide maximum shielding of such areas against external electromagnetic-wave or other interference. The rearward face of each crystal is connected by highly flexible conductor means to the central conductors of the coaxial cables. The forward faces of the respective crystals are preferably grounded through the tips and the jackets electrically connected thereto to the coaxial shields of the cables. Central members or plugs are employed to connect the shield to its respective jacket and at the same time isolate the crystalconnector junction areas.

-III

PATENIED W25 l97| INVENTOR. BEN L. SEEGMILLER FIG.5

FIG.4

HIS ATTORNEY PROBE DEVICE USABLE IN MEASURING STRESS The present invention relates to probes used in stress-measuring equipment and, more particularly, to a new and improved probe device which insures optimum versatility and reliability in use.

A problem in underground mining which has arrested the attention of many individuals and groups is that of properly supporting overburden. The ever-present problem of mine cave-in with its hazards to life, let alone production costs, has stimulated research into rock mechanics. The basic goal in this area is to design equipment that can accurately determine in a reasonably short time the magnitude of stress present in a given mine pillar. While the present invention will be described in terms of mine operations, it will nonetheless be understood that the invention is not restricted in such use but may, on the other hand, be used with any equipment employing electrical pulse, transducer techniques, requiring determination of interprobe, pulse-travel velocity to produce, ulti mately, intelligence bearing upon the stress present in the solid object against which the probes are pressed.

As is disclosed in a pending US. Pat. application entitled Measuring Stress," Ser. No. 658,422, filed Aug. 4, 1967, now abandoned, the present inventor being a co-inventor herein, which application is fully incorporated herein by way of reference, a basic scientific phenomenon utilized by the inventor is that the velocity of ultrasonicwaves through a structural member such as a. rock changes in correspondence with change in stress. Very often this will not be a straight line correlation but rather a characteristic curve for an object of a given constituency. In general, however, it may be stated that the velocity of a given mechanical wave or pulse through a rock, for example, is increased as the stress in the rock is increased.

In usual procedure, a stress-velocity relationship for a particular rock type will be developed in the laboratory by applying known stress to a rock sample by measuring the velocity of pulse travel between a pair of test probes spaced a given distance apart. A stress-velocity relationship will be developed for the particular rock type by incrementally changing the stress and measuring the velocity of a pulse traveling as a surface wave between a pair of spaced probes engaging the rock. The stress velocity relationship will be prepared in the form of a graph. Such a system readily lends itself to electronic feasibility in that where a pair of spaced contact probes incorporate transducers, e.g. piezoelectric crystals proximate and operative with their contact ends, then an electrical pulse sent to one probe will be converted by its crystal into a mechanical pulse transferred to the rock surface.

In the science of wave mechanics, when a mechanical impulse is transferred to a solid formation such asa rock, there are generated thereby three different waves, to wit: a slow surface wave (otherwise known as a Rayleigh wave), a shear wave which is somewhat a faster wave, and a longitudinal or primary wave, also known as a P-wave. The latter is the faster wave and is the wave which will generally be used practicing the invention. Such selection is desired as one should look for the wave of first arrival, the fastest wave, or P-wave.

Thus, the selected mechanical wave generated by such pulse will proceed to the remaining probe, be converted back to an electrical pulse, and there routed to suitable comparator circuitry, forming no part of the present invention, which can measure the time duration between the time that the initial pulse was first generated and sent to the first probe and the time of occurrence of the second pulse generated in the second probe in the manner aforesaid. innumerable circuitry has been developed over the last 30 years which measures time intervals between a pulse echo, as in radar, and initial pulse generation.

The real challenge comes, however, in developing a suitablemultiprobe device which is sufficiently versatile, and, most importantly, which is accurate and free from external, electromagnetic-wave or other interference.

Accordingly, a principal object of the present invention is to provide a new and improved probe device which may be used by or incorporated in stress-measuring equipment.

A further object is to provide a new and useful probe device incorporating transducers proximate their mutually spaced contact tips in such manner that the respective electrical connections between transducer, conductor, and preferably tip are properly shielded.

An additional object is to provide a new and improved probe device wherein maximum mechanical vibration of the incorporated piezoelectric crystals is facilitated.

An additional object is to provide maximum shielding, from extraneous electrical and electromagnetic impulses, those areasof the respective probes at which the transducers thereof are electrically connected to the central conductors of the coaxial cables incorporated by the probes.

A further object is to provide in the subject probe device suitable shielding for connector-crystal junctures completely around such junctures.

A further object is to provide a probe device wherein the spacing between the probes of the device may be accurately varied.

In accordance with the invention the probe device is provided with a handle and a base spacedly mounting a pair of probes. The contact tips of; the probes are preferably chisel shaped and parallel, this to provide for a maximum energy transmission to and from the rock formation the probes are to engage as well as permitting unobstructed wave generation along the surface of the rock being tested. Transducers such as piezoelectric crystals are incorporated in each of the respective probes of the device, and those areas of the crystals which are electrically connected to respective input and output leads are appropriately shielded completely around the junction areas so that external radiations will not effect the operation of the device. The probes are likewise sufficiently spaced apart and'elongate such that external mechanical waves along the probe device itself will not interfere with its operation. The contact tips themselves in addition to being parallel and accurately configured, are grounded back to the coaxial shields ofthe respective coaxial cables. Adjustment apparatusis provided for varying the spacing between the probes when desired.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understoodby reference to the following description, taken in connection with the accompanying drawings in'which:

FIG. I is a perspective view of a probe device incorporating the features of the present invention.

FIG. 2 is a fragmentary, enlarged section taken along the' line 2-2 in FIG. 1.

FIG. 3 is a fragmentary view of the structure of FIG. 2, wherein an alternate spacer cylinder is employed between the piezoelectric crystal of the probe and the plug containedtherein.

FIG. 4 is an enlarged fragmentary section taken along the line 4-4 in FIG. I. i

FIG. 5 is a fragmentary section similar to FIG. 4 but illustrates an optional embodiment of the invention wherein a length of the base of the probe device may be discretely adjusted.

FIG. 6 is a fragmentary section taken along the line'6-6 in FIG. 5.

In FIG. 1 base 10 of probe device PD may either comprise an integral member, or constitute a plural-element part including base member ll'and base segment 12 slideably connected thereto in a manner hereafter indicated. At all events, base 10 includes a central aperture 13 which is countersunk at 14 and also enlarged at 15 for receiving the turned end 16' of grasping handle 17. Screw 32 secures handle 17 to base 10, as shown. Handle 17, will, of source, be used to manipulate the device in the manner desired by the user.

Probes 18 each include mounting member 19, the latter having end 20, see FIG. 2, provided with abutment shoulder 21. End 20 is disposed within aperture 22 of the base 10. Aperture 23 is concentric with respective aperture 22 and is also contiguous therewith at the shoulder 24 against which end 20 seats.

Passing through aperture 23 is a coaxial cable 25. The latter is provided with a central conductor 26 having insulation 27 which is surrounded by the conventional coaxial shield 28. If desired, the coaxial cable may include outer insulation covering the coaxial shield 28.

The coaxial shield 28 of each of the probes 28 is secured by a conductive binder, such as silver solder or a silver epoxy, to conductive plug 29. The latter may be made of copper, aluminum, or other suitable material. Plug 29, in turn, may be silver-soldered at 30 to external-interference-shielding jacket 31, forming in effect a part thereof. Jacket 31 is likewise fabricated from conductive material such as copper or aluminum, by way of example. Mounting member 19 may be made of plastic such as Lucite and can be provided with a recess area 33 terminating in shoulder 34. It is seen that jacket 31 seats in recessed area 33 and abuts shoulder 34.

A nonconductive, plastic or other spacer 35 abuts plug 29 and itself includes a recessed area 36 terminating in a shoulder. Seated within recessed area 36 is a transducer such as piezoelectric crystal 37. The rear surface thereof at 38 receives the forward end of flexible conductor 39 and is electrically connected thereto by a silver solder or silver epoxy by way of example. The forward surface 40 of piezoelectric crystal 37 may be coated with a silver-flake containing epoxy or other bonding agent for securing the rear surface 41 of electrically conductive probe tip 42 to the forward surface of the piezoelectric crystal.

As to flexible conductor 39, the latter may comprise a conventional fine-wire mesh connection. It is essential that this be completely flexible so as not to deter in any way the proper vibration of the piezoelectric crystal 37 in response to electrical energy imparted thereto via conductor 26 and the flexible conductor 39. The spacer 35 will, of course, be made of nonconductive material such as a suitable plastic as seen in FIG. 2 or an elastomeric, insulative material such as rubber, as shown at 35 in FIG. 3.

7 Referring to FIG. 2, the probe tip 42 is maintained at ground or common reference potential, i.e. the potential existing at coaxial shield 28. Such is desirable, for further interference reduction. This can be done by the employment of silver epoxy 43 physically and electrically connecting the tip 42 with jacket 31.

Of importance is the shape of the probe tips 42. It has been found through experimentation that the chisel-configured edges 44 are most satisfactory in use. These should be parallel and flat on their ends, howbeit of reduced transverse dimension of the order of three-sixteenths", for example. In this way there can be a maximum transfer of energy from a tip 42 of one of the probes 18, where maximum reception of the desired energy wave by the remaining probe tip and probe is desired. It is here to be mentioned that, preferably, both the probes 18 are constructed in similar fashion as shown in FIGS. 2 and 3, with both of the same being mounted in the respective apertures 22 in the manner indicated in FIG. 2.

The operation of the probe device PD as thus far described is as follows.

An external source of electrical energy, not shown, supplies a pulse of electrical energy to central conductor 26 of coaxial cable of a given one of the probes. This pulse will take the form of a voltage difference of specific time duration as between conductor 26 and the coaxial shield 28; in the system the latter is preferably maintained at a ground or common reference potential, for proper shielding effect. It is noted in FIG. 2 that the tip 42 also is grounded to the coaxial shield of the coaxial cable-25 through the tip 42, solder or epoxy 43, jacket 31, and the electrical connection of the latter through solder to plug 29. The latter is, of course, physically and electrically connected to the coaxial shield 28 via silver epoxy 44. Thus, in referring to FIG. 2, and considering the upper probe 18 of FIG. 1 to be the input probe, this pulse of electrical energy is transmitted to the piezoelectric crystal, which by its nature, and in a manner well known in the art, converts such electric impulse into a physical vibration pulse. This pulse of mechanical energy is immediately-transmitted to tip 42 and through the same to the rock formation or other structure being tested. The first-arrival or P-wave produced in the formation travels to the remaining probe via the formation which it touches, and the mechanical energy is converted by piezoelectric crystal 37 in the remaining probe to electrical energy, the latter traveling through flexible conductor 39 and conductor 26 of the remaining probe, back through the remaining probe 18 to external detector equipment. It would be understood that the pulse generating equipment and detector equipment form no part of the present invention.

It is most important to note that by virtue of the construction of the present invention, see specifically FIG. 2, the area within spacer 35 is completely shielded from exterior influence.

For experimentation or other purposes it maybe desirable to have the parallel tip edges 44 of the probes variably spaced apart. This can be accomplished by constructing base 10 as a plural element unit in the manner heretofore indicated in connection with FIG. 1. Where such is the case, then base member 11 and base segment 12 may be configured in the manner shown in FIG. 4. In such event, base member 11 is provided with slot 46 in a downward extension 47. Correspondingly, base segment 12 may be provided with an upward extension 48 is slideably disposed and cooperative with downward extension 47. Screw 49 is provided with washer 50 and is threaded into threaded aperture 51 of upper extension 48. This screw, of course, is disposed in slot 46.

By virtue of such construction, the loosening of the screw will provide for slightly slideable adjustment of base segment 12 relative to base member 11 so that the chisel-shaped ends 44 of the probe may be variably and adjustably spaced apart.

FIGS. 5 and 6 disclose another type of adjustable construction wherein the upward extension 48' corresponding to upward extension 48 in FIG. 4 is this time provided with apertures 52 and 53 for receiving screw 54, and stud 55, respectively. Keyed to screw 54 is a pinion 56, and the end of the screw 54 is provided with a retainer 57 for retaining the end in keyed block 58. Rack segment 59 may be secured in any conventional manner to base member 11, corresponding to base member 11 in FIG. 4, at the slot area 46' corresponding to 46 in FIG. 4. Knurled nut 60 is threaded upon the threaded shank 61 of stud 55 where engages the slide block 58 in the manner seen in FIG. 5. I l

The manner of adjustment of the structure is seen in FIGS. 5 and 6 as follows. Knurled nut 60 is first unthreaded so as to release the interlocking of the parts. Screw 54 is then turned by head 62 so as to rotate pinion 56. Such rotation of pinion 56 and its engagement with rack 59 provides a downward traveling of the pinion along the rack and hence a carrying of the part 12, corresponding to part 12 in FIG. 4, downwardly relative to part 11'. Upon a sufficient separation of the ends 44 of probes 18 being achieved, then the turning of screw 54 is ceased and the disposition locked in place by the threading down of nut 60 against slide block 58.

Hence, the construction of FIG. 4 and of FIGS. 5 and 6 represent different embodiments suggesting ways in which the contact edges 44 of probes 18 in FIG. 1 may be adjustably spaced relative to each other. This adjustability feature in spacing may well be desirable in certain instances so as to correlate any electronic equipment and any limitations thereof to the particular formation and velocity of wave travel anticipated.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects.

I claim:

1. For a stress-measuring system, including a readout device, that sends and receives pulsed mechanical waves between two probes in contact with the sample whose stress is being measured, such that the time delay between transmission and reception of such pulses is measured in the readout device, indicating the actual stress present in said sample, a probe device including, in combination, a pair of mutually spaced probes, and means for mounting said probes in mutually spaced relationship, each of said probes including: an electrically conductive contact tip, a piezoelectric crystal having a front surface engaging said contact tip and a rear surface, and a coaxial cable having a central conductor and an electrically conductive coaxial shield circumscribing and insulated from said central conductor, said coaxial shield being electrically grounded, means for electrically coupling said central conductor to said rear surface of said piezoelectric crystal, and electrically conductive means encasing said piezoelectric crystal and the juncture thereof with said central conductor for forming an enlarged, grounded continuation of said coaxia1 shield thereabout, said electrically conductive means being coupled electrically to said tip, whereby to be electrically coupled to and between said front surface of said piezoelectric crystal and said coaxial shield, said coaxial shield, electrically conductive encasing means, and contact tip completely shieldingly enclosing said piezoelectric crystal and said central conductor where connected thereto, to electrically shield the same from external interference.

2. For a stress-measuring system, including a readout device, that sends and receives pulsed mechanical waves between two probes in contact with the sample whose stress is being measured, such that the time delay between transmission and reception of such pulses is measured in the readout device, indicating the actual stress present in said sample, a probe device including, in combination, a pair of mutually spaced probes, and means for mounting said probes in mutually spaced relationship, each of said probes including: an electrically conductive contact tip, a coaxial cable having a central conductor and a coaxial shield circumscribing and insulated from said central conductor, and ultrasonic electromechanical transducer means interposed between and operatively coupled to said central conductor and said contact tip for, as to one probe, converting electrical energy received by its central conductor into mechanical vibration for said probe s contact tip and, as to the remaining probe, for receiving mechanical vibration from its respective contact tip and converting the same into electrical energy for its respective central conductor, and respective, electrically conductive, shielding means circumscribing said ultrasonic electromechanical transducer and said central conductor and being electrically connected to said respective coaxial shield and also to the tip, said coaxial shield, electrically conductive shielding means, and contact tip completely shieldingly enclosing said piezoelectric crystal and said central conductor where connected thereto, to electrically shield the same from external interference. 

1. For a stress-measuring system, including a readout device, that sends and receives pulsed mechanical waves between two probes in contact with the sample whose stress is being measured, such that the time delay between transmission and reception of such pulses is measured in the readout device, indicating the actual stress present in said sample, a probe device including, in combination, a pair of mutually spaced probes, and means for mounting said probes in mutually spaced relationship, each of said probes including: an electrically conductive contact tip, a piezoelectric crystal having a front surface engaging said contact tip and a rear surface, and a coaxial cable having a central conductor and an electrically conductive coaxial shield circumscribing and insulated from said central conductor, said coaxial shield being electrically grounded, means for electrically coupling said central conductor to said rear surface of said piezoelectric crystal, and electrically conductive means encasing said piezoelectric crystal and the juncture thereof with said central conductor for forming an enlaRged, grounded continuation of said coaxial shield thereabout, said electrically conductive means being coupled electrically to said tip, whereby to be electrically coupled to and between said front surface of said piezoelectric crystal and said coaxial shield, said coaxial shield, electrically conductive encasing means, and contact tip completely shieldingly enclosing said piezoelectric crystal and said central conductor where connected thereto, to electrically shield the same from external interference.
 2. For a stress-measuring system, including a readout device, that sends and receives pulsed mechanical waves between two probes in contact with the sample whose stress is being measured, such that the time delay between transmission and reception of such pulses is measured in the readout device, indicating the actual stress present in said sample, a probe device including, in combination, a pair of mutually spaced probes, and means for mounting said probes in mutually spaced relationship, each of said probes including: an electrically conductive contact tip, a coaxial cable having a central conductor and a coaxial shield circumscribing and insulated from said central conductor, and ultrasonic electromechanical transducer means interposed between and operatively coupled to said central conductor and said contact tip for, as to one probe, converting electrical energy received by its central conductor into mechanical vibration for said probe''s contact tip and, as to the remaining probe, for receiving mechanical vibration from its respective contact tip and converting the same into electrical energy for its respective central conductor, and respective, electrically conductive, shielding means circumscribing said ultrasonic electromechanical transducer and said central conductor and being electrically connected to said respective coaxial shield and also to the tip, said coaxial shield, electrically conductive shielding means, and contact tip completely shieldingly enclosing said piezoelectric crystal and said central conductor where connected thereto, to electrically shield the same from external interference. 